<form action="" method="post">
<textarea id="mitexto"></textarea>
<input type="submit" value="enviar">
</form>
<script>
document.querySelector("input[type=submit]").addEventListener("click",function(e){
var texto=document.getElementById("mitexto")
if(texto.value.length<10)
{
e.preventDefault();
console.log("Tiene menos de 10 caracteres");
}
});
</script>